part 1] DISTRICT AROUND CORRIS AUD ABERLLEFENNI. 533 



Of these two anticlinal structures, the larger and more important 

 is the Corris Anticline. It is the northward continuation of the 

 Cef n Maesmawr Anticline, which was described in the Machynlleth 

 area (see fig. 5, p. 534). Although this anticlinal structure persists 

 right through from the River Dyii to Corris, its character changes 

 considerably northwards. Immediately south of the Dyfi, the 

 Cefn Maesmawr Anticline is overfolded towards the east, and the 

 eastern limb is in most places replaced by a well-marked strike- 

 fault which has been called the Brwyno Overthrust. At 

 Corris this overthrust has disappeared, and, in addition, the anti- 

 clinal structure is not markedly asymmetrical. 



The character of the Corris Anticline is well brought out by the 

 outcrops of the various bands which have been mapped : as, for 

 instance, the zone of Gtlyptograptus persculptus at Cwm Cadian 

 and the thick grit-band in the G-arnedd-wen Beds on Mynydd 

 Braich-goch, etc. The nose of the anticline is particularly well seen 

 in the southernmost Braich-goch slate-quarry (see fig. 4, p. 532). 



The anticlinal structure seems scarcely to make itself felt on the 

 outcrop of the Nod Grlas, and this feature has been observed, in 

 addition, at several localities outside the area at present described. 

 While the outcrops of the Pont Erwyd Stage and the Abercorris 

 Croup may indicate pronounced folding, this is rarely revealed by 

 the outcrop of the Nod Grlas, which runs in a general south-westerly 

 and north-easterly dii-ection with remarkably few deviations. 



This curious variation in folding in different beds is difficult to 

 explain, unless it be that the folds diminish in intensity north- 

 wards, or that the massive, compact Abercwmeiddaw Mudstones 

 protect the Nod Glas from the repeated folding experienced by the 

 higher beds. 



It is probably the Corris anticlinal axis which brings in the 

 outcrop of the volcanic rocks on the north-western side of Mynydd 

 y Waun. At the latter place the western limb of the structure is 

 replaced by a well-marked strike-fault. 



The Aberllefenni Anticline is not so important a structural 

 feature as the Corris Anticline. It is however, in all probability, the 

 northward continuation of the llhiwlwyfan Anticline recorded at 

 Machynlleth (see fig. 5). There is no indication of the Rhiwlwyfan 

 Overthrust which marks the eastern limb of the anticline farther 

 south, nor is the Aberllefenni Anticline markedly asymmetrical. 

 The anticlinal structure appears to die out immediately north of 

 Aberllefenni, for there is no indication of the fold in the outcrops 

 of the Narrow Vein and the lied Vein as they cross over the ridge 

 of Foel Crochan, north of Aberllefenni ; in fact, the fold appears to 

 die out so soon as the Aberllefenni strike-fault is reached. 



The small synclinal structure which intervenes between the 

 above-mentioned anticlines is the northern extension of the 

 G-laspwll Syncline at Machynlleth; while the broad synclinorium 

 west of the Corris anticlinal axis may be correlated with the 

 synclinorium farther south, the eastern and western limbs of which 



